Chapter 4 Traffic Engineering Studiessite.iugaza.edu.ps/wp-content/uploads/file/ECIV43332012/Ch 04 Traf… · Traffic Engineering Studies Chapter 4 Traffic Engineering Studies Traffic

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Traffic Engineering Studies

Chapter 4


Traffic studies may be grouped into three main categories: (1) Inventories, (2) Administrative studies, and (3) Dynamic studies.

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(1) Inventories:provide a list or graphic display of existing information, such as: street widths, parking spaces, transit routes, traffic regulations.


(2) Administrative studies use existing engineering records,

available in government agencies and departments.

include the results of surveys, which may involve: field measurements and/or aerial photography.

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(3) Dynamic traffic studies involve the collection of data under

operational conditions and include studies of:

speed, traffic volume, travel time and delay, parking, and crashes.

They are described in detail in this chapter.


4.1 SPOT SPEED STUDIES

Spot speed studies are conducted to estimate the distribution of speeds of vehicles in a stream of traffic at a particular location on a highway.

carried out by recording the speeds of a sample of vehicles at a specified location.

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SPOT SPEED STUDIES

Used to: Establish parameters for

traffic operation and control, such as: speed zones, speed limits (85th-percentile speed)


4.1.1 Locations for Spot Speed Studies Represent different traffic conditions on a

highway for basic data collection. Mid-blocks of urban highways and straight,

level sections of rural highways for speed trend analyses.

Any location may be used for solution of a specific traffic engineering problem.

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4.1.1 Locations for Spot Speed Studies

Should be selected to achieve the following: Unbiased data Drivers be unaware Equipment concealed from the driver, Observers inconspicuous.


4.1.1 Locations for Spot Speed Studies statistical analysis, statistically adequate number of vehicle

speeds be recorded.

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4.1.2 Time of Day and Duration of Spot Speed Studies

depends on the purpose of the study. recommended when traffic is free-flowing, during off-peak hours. typically:

the duration is at least 1 hour and the sample size is at least 30 vehicles.


4.1.3 Sample Size for Spot Speed Studies The larger the sample size, will give an

estimated mean within acceptable errorlimits. Average Speed Median Speed Modal Speed The ith-percentile Spot Speed Pace Standard Deviation of Speeds

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4.1.4 Methods for Conducting Spot Speed Studies manual and automatic manual method is seldom used automatic devices

1. road detectors2. radar-based3. the principles of electronics.


Road Detectors pneumatic road tubes & induction loops

collect data on speeds & volume at the same time

Advantage: Human errors are considerably reduced

Disadvantages: expensive may, affect driver behavior,

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Pneumatic road tubes laid across the lane in which data are to

be collected. When moving vehicle passes over, an air

impulse is transmitted to the counter. two tubes are placed across the lane, 2 m

apart. An impulse is recorded when the front

wheels of a moving vehicle pass over the first tube;


Pneumatic road tubes a second impulse is recorded when the

front wheels pass over the second tube. The time elapsed between the two

impulses and the distance between the tubes are used to compute the speed of the vehicle.

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inductive loop a rectangular wire loop buried under the

roadway surface. It operates on the principle that a

disturbance in the electrical field is created when a motor vehicle passes across it.


Radar-Based Traffic Sensors Electronic-Principle Detectors

traffic characteristics, such as speed, volume, queues, and headways are computed.

Using video image processing

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(a) RTMS Deployed in the Forward Looking Mode


(b) RTMS Deployed in the Side-fire Mode

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(a) Schematic Illustration of the Auto scope


(b) The Auto scope Deployed

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4.1.5 Presentation and Analysis of Spot Speed Data Statistical methods Analyzing data frequency histogram cumulative frequency distribution curve


Example 4.2 Determining Speed Characteristics from a Set of Speed Data. Table 4.2 shows the data collected on a rural highway in Virginia during a speed study. Develop the frequency histogram and the frequency distribution of the data and determine:

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1. The arithmetic mean speed2. The standard deviation3. The median speed4. The pace5. The mode or modal speed6. The 85th-percentile speed

Traffic Engineering Studies Solution: The speeds range from 34.8 to 65.0 km/h,

giving a speed range of 30.2. For eight classes, the range per class is 3.75

km/h; for 20 classes, the range per class is 1.51 km/h. It is convenient to choose a range of 2 km/h per

class which will give 16 classes. A frequency distribution table can then be

prepared, as shown in Table 4.3.

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Table 4.2 Speed Data Obtained on a Rural Highway


Figure 4.4 Histogram of Observed Vehicles' Speeds

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Table 4.3 Frequency Distribution Table for Set of Speed Data


Figure 4.5 Frequency Distribution

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Figure 4.6 Cumulative Distribution


The median speed 49 km/h, the 50th-percentile speed.

85th-percentile speed is 54 km/h

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4.2 VOLUME STUDIES1. Average Annual Daily Traffic (AADT)

the average of 24-hour counts collected every day of the year.

2. Average Daily Traffic (ADT) the average of 24-hour counts collected over a number of days greater than one but less than a year.


4.2 VOLUME STUDIES3. Peak Hour Volume (PHV)

the maximum number of vehicles that pas a point on a highway during a period of 60 consecutive minutes.

4. Vehicle Classification (VC) with respect to the type of vehicles for cars, two-axle trucks, or three-axle trucks.

5. Vehicle Miles of Travel (VMT)

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4.2.1 Methods of Conducting Volume Counts Manual Method Automatic Method


Figure 4.7 Jamar Traffic Data Collector TDC-1 2 Hooked to a Computer

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Figure 4.9 Apollo Traffic Counter/Classifier


Figure 4.10 Example of Counters that Require the Laying of Subsurface Detectors

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Figure 4.11 Traffic Eye Universal System


Figure 4.12 Example of Station Locations for a Cordon Count

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4.2.2 Types of Volume Counts Depending on the anticipated use of the

data to be collected. Intersection Counts

vehicle classifications, through movements, turning movements.


4.2.2 Types of Volume Counts Pedestrian Volume Counts Periodic Volume Counts (AADT)

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4.2.3 Traffic Volume Data Presentation Traffic Flow Maps:

volume of traffic on each route is represented by the width of a band. Figure 4.13 shows a typical traffic flow map.

Figure 4.13 Example of a Traffic Flow Map


Figure 4.14 Intersection Summary Sheet

Intersection Summary Sheets:

Figure 4.14 shows a typical intersection summary sheet.

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4.2.3 Traffic Volume Data Presentation Time-Based Distribution Charts:

see Figure 4.15


Figure 4.15 Traffic Volumes on an Urban Highway (A&B)

Daily variations: see Figure

4.15b

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Figure 4.15 Traffic Volumes on an Urban Highway (C)

Hourly variations in traffic volume:


Table 4.4 Summary of Traffic Volume Data for a Highway Section

Summary Tables: PHV, Vehicle Classification (VC), and

ADT. See Table 4.4

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Adjustment of Periodic Counts Expansion Factors from Continuous

Count Stations. Hourly expansion factors (HEFs) are

determined by the formula


Daily expansion factors (DEFs) are computed as

Monthly expansion factors (MEFs) are computed as

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Table 4.5 Hourly Expansion Factors for a Rural Primary Road


Table 4.6 Daily Expansion Factors for a Rural Primary Road

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Traffic Engineering StudiesTable 4.7 Monthly Expansion Factors for a Rural Primary Road


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4.3 TRAVEL TIME AND DELAY STUDIES Travel time: time required to travel from

one point to another on a given route. the locations, durations, and causes of

delays. good indication of the level of service identifying problem locations,

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4.3.1 Applications of Travel Time and Delay Data efficiency of a route locations with relatively high delays causes for delays before-and-after studies relative efficiency of a route travel times on specific links economic studies


4.3.2 Definition of Terms Related to Time and Delay Studies

1. Travel time: time taken by a vehicle to traverse a given section of a highway.

2. Running time: time a vehicle is actually in motion

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Traffic Engineering Studies 4.3.2 Definition of Terms Related to Time

and Delay Studies

3. Delay time lost due to causes beyond the control of the driver.

4. Operational delay: delay caused by the impedance of other traffic. (for example, parking or unparking vehicles),


5. Stopped-time delay6. Fixed delay: caused by control devices

such as traffic signals, regardless of the traffic volume

7. Travel-time delay: difference between the actual travel time and the travel time obtained by assuming that a vehicle traverses at an average speed equal to that for an uncongested traffic flow

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4.3.3 Methods for Conducting Travel Time and Delay Studies Methods Requiring a Test Vehicle:

floating-car, average-speed, and moving-vehicle techniques.

Traffic Engineering StudiesFloating-Car Technique: test car is driven by an observer along the test

section. The driver attempts to pass as many

vehicles as those that pass his test vehicle.

Time taken to traverse the study section is recorded. This is repeated, and the average time is recorded as the travel time.

sample size s usually less than 30,

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Average-Speed Technique. driving the test car along the length of the

test section at a speed that, is the average speed of the traffic stream. time required to traverse the test section is

noted. test run is repeated the average time is recorded as the travel

time.


Average-Speed Technique. travel time is usually obtained the observer starts a stopwatch at the

beginning point of the test section and stops at the end.

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Traffic Engineering StudiesTable 4.8 Speed and Delay Information


Average-Speed Technique. A second stopwatch also may be used to

determine the time that passes each time the vehicle is stopped.

will give the stopped-time delay Table 4.8 shows an example of a set of

data obtained for such a study.

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Moving-Vehicle Technique (moving observer): the observer makes a round trip on a test

section Figure 4.16, The observer starts at section X-X, drives

the car eastward to section Y-Y, turns the vehicle around drives westward to section X-X again


Figure 4.16 Test Site for Moving-Vehicle Method

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Moving-Vehicle Technique. following data are collected as

The time it takes to travel east from X-X to Y-Y (Te), in minutes

The time it takes to travel west from Y-Y to X-X (Tw), in minutes

The number of vehicles traveling west in the opposite lane while the test car is traveling east (Ne)


Figure 4.16 Test Site for Moving-Vehicle Method

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Moving-Vehicle Technique. The number of vehicles that overtake the

test car while it is traveling west from Y-Y to X-X, that is, traveling in the westbound direction (Ow)

The number of vehicles that the test car passes while it is traveling west from Y-Y to X-X, that is, traveling in the westbound direction (Pw)


Moving-Vehicle Technique. The volume (Vw) in the westbound

direction can then be obtained from the expression:

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where (Ne Ow Pw) is the number of vehicles traveling westward that cross the line X-X during the time (TeTw).

Similarly, the average travel time in the westbound direction is obtained from


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Methods Not Requiring a Test Vehicle

License-Plate Observations: observers at the beginning and end of the test section.

Each observer records the last three or four digits of the license plate of each car that passes, together with the time at which the car passes.


in the office by matching the times of arrival at the beginning and end of the test section for each license plate recorded.

difference between these times is the traveling time of each vehicle.

average of these is the average traveling time on the test section.

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a sample size of 50 matched license plates.

Interviews: obtaining information from people who drive on the study site regarding their travel times, experience of delays, requires the cooperation of the people.


ITS Advanced Technologies: Advanced technologies Cell phones GPS satellite system technology is used to determine average

speeds and travel times along highways

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4.4 PARKING STUDIES Any vehicle will at one time be parked

short time or longer time, provision of parking facilities is essential

need for parking spaces is usually very great in areas of business, residential, or commercial activities.

park-and-ride


Providing adequate parking space to meet the demand for parking in the Central Business District (CBD)

This problem usually confronts a city traffic engineer.

solution is not simple, Parking studies are used to determine the demand for and the supply of parking facilities.

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4.4.1 Types of Parking Facilities On-Street Parking Facilities

also known as curb facilities. Parking bays are provided alongside the curb on one or both sides of the street.

unrestricted parking unlimited and free Restricted parking facilities


On-Street Parking Facilities limited to specific times for a maximum

duration. may or may not be free. handicapped parking bus stops loading bays.

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Off-Street Parking Facilities privately or publicly owned; surface lots and garages. Self-parking garages attendant-parking garages


4.4.2 Definitions of Parking Terms1. A space-hour is a unit of parking that

defines the use of a single parking space for a period of 1 hour.

2. Parking volume is the total number of vehicles that park in a study area during a specific length of time, usually a day.

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3. Parking accumulation is the number of parked vehicles in a study area at any specified time.

4. parking load the number of space-hours used during the specified period of time.

5. Parking duration length of time a vehicle is parked at a parking

indication of how frequently a parking space becomes available.


6. Parking turnoverrate of use of a parking space.

Obtained by dividing the parking volume for a specified period by the number of parking spaces.

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4.4.3 Methodology of Parking Studies Inventory of Existing Parking Facilities

detailed listing of the location and all other relevant characteristics of each legalparking facility, private and public.

The study area includes both on- and off-street facilities.

Traffic Engineering Studies Type and number of parking spaces at each

parking facility Times of operation and limit on duration of parking,

if any Type of ownership (private or public) Parking fees, method of collection Restrictions Other restrictions, loading and unloading zones,

bus stops, taxi ranks Permanency

The inventory should be updated at regular intervals of about four to five years.

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4.4.3 Methodology of Parking Studies Collection of Parking Data

Accumulation: by checking the amount of parking during

regular intervals on different days of the week. Carried out on an hourly or 2-hour basis used to determine hourly variations of parking

and peak periods of parking demand.


Collection of Parking Data Turnover and Duration:

collecting data on a sample of parking spaces in a given block.

recording the license plate of the vehicle parked on each parking space in the sample at the ends of fixed intervals during the study period.

The length of the fixed intervals depends on the maximum permissible duration.

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Turnover and Duration: For example, if the maximum permissible

duration of parking at a curb face is 1 hour, a suitable interval is every 20 minutes.

If the permissible duration is 2 hours, checking every 30 minutes would be appropriate. Turnover is then obtained from the equation


Figure 4.17 Parking Accumulation at a Parking Lot

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Turnover and Duration: manual collection of parking data is still

commonly used, Possible for all parking data to be collected

electronically. wireless sensors

Identification of Parking Generators (for example, shopping centers or transit

terminals) and locating these on a map of the study area.


Parking Demand by interviewing drivers at the various parking

facilities Interview all drivers using the parking facilities

on a typical weekday between 8:00 a.m. and 10:00 p.m.

Information include (1) trip origin, (2) purpose of trip, (3) driver’s destination after parking.

the location of the parking facility, times of arrival and departure, vehicle type.

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Parking Demand Parking interviews also can be carried out

using the postcard technique, about 30 to 50 percent of the cards distributed

are returned.


4.4.4 Analysis of Parking Data Number and duration for vehicles legally

parked Number and duration for vehicles illegally

parked Space-hours of demand for parking Supply of parking facilitiesThe space-hours of demand for parking are obtained from the expression

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4.4.4 Analysis of Parking Data

whereD = space vehicle-hours demand for a specific period of timeN = number of classes of parking duration rangesti = midparking duration of the ith classni = number of vehicles parked for the ith duration rangeThe space-hours of supply are obtained from the expression



whereD = space vehicle-hours demand for a specific period of timeN = number of classes of parking duration rangesti = midparking duration of the ith classni = number of vehicles parked for the ith duration rangeThe space-hours of supply are obtained from the expression

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whereS = practical number of space-hours of supply for a specific period of timeN = number of parking spaces availableti = total length of time in hours when the ith space can be legally parked on during the specific periodf = efficiency factor


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Chapter 4 Traffic Engineering Studiessite.iugaza.edu.ps/wp-content/uploads/file/ECIV43332012/Ch 04 Traf… · Traffic Engineering Studies Chapter 4 Traffic Engineering Studies Traffic